The present invention relates to a method for the modernisation of a plant for urea production according to the process with stripping with carbon dioxide.
More specifically, the invention relates to a method for the modernisation of a plant for urea production of the type comprising:
a reactor for urea synthesis;
means for feeding ammonia and carbon dioxide to the reactor for urea synthesis;
a stripping unit with carbon dioxide for subjecting a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to a treatment of partial decomposition of carbamate and partial separation of free ammonia, thus obtaining a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising urea and residual carbamate in aqueous solution, respectively;
a recovery section for the flow comprising urea and residual carbamate in aqueous solution leaving the stripping unit for separating urea from the residual carbamate in aqueous solution;
at least one vertical condensation unit of the film type, comprising a tube bundle for subjecting to partial condensation the flow comprising ammonia and carbon dioxide in vapour phase leaving the stripping unit, thus obtaining a liquid flow comprising carbamate in aqueous solution and a gaseous flow comprising ammonia and carbon dioxide in vapour phase;
means for respectively feeding the flow comprising carbamate in aqueous solution and the flow comprising ammonia and carbon dioxide in vapour phase to the reactor for urea synthesis;
means for feeding a gaseous flow comprising one or more passivating agents, inert with respect to the synthesis reaction of urea, to the stripping unit and from the stripping unit to the reactor for urea synthesis through the condensation unit.
In the following description and enclosed claims, with the term xe2x80x9cmodernisationxe2x80x9d, it is intended to mean the modification of a pre-existing plant with the purpose of improving its performance and obtaining, for example, an increase of the production capacity and/or of the conversion yield as well as a reduction of the energy consumption. In particular, the modernisation of a urea plant according to the present invention foresees to increase the capacity, maintaining the main equipment of the high pressure synthesis section (synthesis loop).
In the following description and enclosed claims, with the term xe2x80x9ccondensation unit of the film typexe2x80x9d, it is intended to mean an apparatus wherein the condensation of the gaseous phase occurs in a liquid film, flowing downwards inside a plurality of tubes in co-current with the gaseous flow. The liquid film flows in contact with the tube wall whereas the gaseous phase flows inside the tubes.
According to a further aspect of thereof, the present invention also relates to a process for urea production as well as to a plant for carrying out such process.
As known, with respect to urea production, the need is more and more felt of having on one hand plants of always greater capacity and operation efficiency available, and on the other hand of having lower and lower investment and operation costs, as well as lower energy consumption.
To this end, methods for the modernisation of existing plants for urea production according to the stripping process with carbon dioxide have been recently set forth in the field, substantially based on the modification of the synthesis reactor, on the replacement of the apparatuses downstream the synthesis reactor with apparatuses of greater capacity and/or on the addition of new apparatuses in parallel to the existent apparatuses.
For example, in EP-A-0 796 244 a method of modernisation of a plant for urea production is disclosed, which foresees the addition of a partial decomposition step of the carbamate in aqueous solution recycled to the synthesis reactor. With this method of modernisation, it is possible to remarkably reduce the amount of water recycled to the synthesis reactor, thus permitting to obtain an increase of the conversion yield and therefore of the production capacity of the plant.
One of the main problems that is encountered when an increase of production capacity is considered in existing plants for urea production according to a stripping process with carbon dioxide, is that of increasing the capacity of the high pressure condensation section which may comprise one or more condensation units.
The methods of modernisation for increasing the capacity of said condensation section proposed to date always foresee the insertion in parallel of additional condensation units of the film type or of a unit provided with a high exchange coefficient, for example a horizontal condensation unit of the Kettle type (pool condenser).
In some instances the replacement of the existing unit(s) with new units of greater capacity must be even taken into account.
Such provisions have a very negative impact both on investment costs and on the constructional complexity relative to the modernisation of the condensation section according to the above mentioned methods of the prior art.
Because of these disadvantages, the modernisation of plants for urea production according to the stripping process with carbon dioxide has found to date a relatively reduced application, notwithstanding the ever increasing interest of industry of modifying the existing plantsxe2x80x94instead of replacing them with new plantsxe2x80x94with the purpose of increasing the production capacity and decreasing the energy costs.
The technical problem underlying the present invention is therefore that of providing a method for the modernisation of a plant for urea production which allows an increase of the production capacity thereof, implies low energy consumption and investment costs and is technically easy to be implemented.
According to the present invention, this problem is solved by a method of the aforementioned type, which is characterised by comprising the steps of:
providing means for feeding a minor portion of the flow comprising ammonia and carbon dioxide in vapour phase together with a minor portion of the gaseous flow comprising one or more passivating agents leaving the stripping unit directly to the reactor for urea synthesis;
providing means for feeding a major portion of the flow comprising ammonia and carbon dioxide in vapour phase together with a major portion of the gaseous flow comprising one or more passivating agents leaving the stripping unit to such at least one condensation unit;
providing in such at least one condensation unit means for subjecting to substantially total condensation the major portion of the flow comprising ammonia and carbon dioxide in vapour phase, obtaining a flow comprising urea and carbamate in aqueous solution.
As far as the term major portion is concerned, this indicates a portion greater than 50% of the total flow of ammonia and carbon dioxide in vapour phase coming out of the stripping unit. Depending on the operating conditions in the synthesis reactor, the major portion can be in a range between 65 and 85% of such flow, for example between 70 and 75%.
Advantageously, the present invention permits to remarkably increase the exchange coefficient and therefore the efficiency of the condensation section, permitting a debottlenecking of the existing plant to full advantage of the overall production capacity, which may be therefore optimally increased.
All this is attained in a simple and effective way, with minimum and quite secondary interventions in the high pressure synthesis section that is thus maintained substantially unchanged, and with low energy consumption.
Accordingly the investment, implementation and operation costs are considerably lower than the costs required by the methods of modernisation according to the prior art.
In fact, thanks to the present method, the pre-existing condensation section is not upgraded nor replaced with new apparatuses, but advantageously preserved, requiring only small internal modifications of the condensation unit(s) in such a way to obtain a substantially total condensation of the gaseous phase fed therein.
Thanks to the fact of providing means for splitting respectively in a minor and major portion the flow comprising ammonia and carbon dioxide in vapour phase (coming from the stripping unit), the portion of reactants in gaseous phase necessary for controlling the reaction temperature inside the synthesis reactor is not made any longer to pass through the condensation unit together with the liquid phase, as in the prior art. So doing, the condensation unit can be modified internally in such a way to permit the substantial condensation of all the gaseous reactants comprised in the major portion and hence to operate at the maximum efficiency obtainable.
In this respect, the present method of modernisation advantageously comprises the step of providing means for transforming the vertical condensation unit of the film type in a vertical condensation unit of the xe2x80x9csubmergedxe2x80x9d type.
In the following description and in the enclosed claims, with the term xe2x80x9ccondensation unit of the submerged typexe2x80x9d, it is intended to mean an apparatus wherein the liquid phase fills (submerges) the tube bundle and wherein the condensation of the gaseous phase occurs by passing through such liquid phase. In other words, in this case, the condensation unit operates having the tube bundle""s tubes full of liquid, differently from the condensation unit of the film type wherein the tubes are substantially empty.
More in particular, the existing condensation unit is advantageously modified by providing means for feeding the major portion of the flow comprising ammonia, carbon dioxide in vapour phase in a lower end of the tube bundle of such at least one unit and means for circulating a flow of condensation liquid comprising carbamate in aqueous solution and ammonia inside the condensation unit with a thermosiphon like motion.
Thanks to the present invention, the vertical position and the dimension of the existing condensation unit are exploited, which guaranteexe2x80x94once transformedxe2x80x94a high liquid head (height) and therefore a natural circulation of the same inside the tube bundle (thermosiphon like motion).
The circulation according to a thermosiphon like motion of the liquid phase is due to the difference of specific weight between the flow comprising liquid and vapours flowing upwards in the apparatus and the flow of liquid only directed downwards through respective portions of the tube bundles.
In this respect, particularly satisfying results have been obtained providing a gas distributor proximate to the lower end of the tube bundle in fluid communication with the means for feeding the major portion of the flow comprising ammonia and carbon dioxide in vapour phase, so as to distribute the ammonia and carbon dioxide in vapour phase in a preferably central, predetermined portion of the tube bundle.
In this way, the condensation of ammonia and carbon dioxide in vapour phase is made advantageously occur only in a well defined portion of the tube bundle, thus promoting an optimal circulation in a thermosiphon like motion of the liquid phase inside the condensation unit between the portion of the tube bundle through which the low specific weight flow passes (gaseous phase mixed with the liquid phase) and the portion of tube bundle (preferably in a peripheral position) through which the high specific weight flow passes (liquid phase only).
With respect to a condensation unit of the film type, the condensation unit provided by the present method of modernisation permits an effective mixing of the gaseous phase in the liquid phase obtaining in this way a remarkable increase of the exchange coefficient inside the tubes and therefore increasing the overall exchange coefficient and the efficiency of the pre-existing condensation section.
This allows to increase the capacity of the existing condensation section.
Test results have shown that it is even possible to double the overall exchange coefficient of the existing condensation unit.
Further, thanks to the step of modifying the vertical condensation unit(s) from the film type to the xe2x80x9csubmergedxe2x80x9d type, with thermosiphon like circulation of the liquid phase, it is possible to increasexe2x80x94in a simple and effective wayxe2x80x94the residence time in such units of the therein provided carbamate, which can thus react and be partially transformed in urea.
In doing so, it is possible to obtain an increase even equal to 10-20%, of the overall reaction volume, that is of the residence time of the carbamate in the condensation unit and in the reaction space. This factor advantageously implies a corresponding increase of the conversion yield of the carbon dioxide into urea.
In order to extract from the condensation section the flow comprising carbamate in aqueous solution and urea so obtained without having to bring remarkable modifications to the original structure, the method of modernisation according to the present invention comprises advantageously the provision in the condensation unit(s) of means for collecting and feeding the aforesaid flow from an area above the tube bundle to a lower end of the condensation unit in fluid communication with the means for feeding the liquid phase to the reactor for urea synthesis.
It is important to notice how, contrary to the recurrent teaching of the prior art, that suggests to upgrade the condensation section adding new condensation units or replacing the existing one with a new unit with a high exchange coefficient, the method of modernisation according to the present invention permits to achieve the same (if not greater) capacity increases with the existing condensation unit(s), which are advantageously only slightly modified from the structural point of view but are dramatically changed as far as their operation is concerned.
This result is absolutely surprising if we consider that according to the methods of modernisation of the prior art it was not conceivable to obtain substantial capacity increases in the condensation section with the existing apparatuses only.
According to a particularly advantageous and preferred embodiment of the present invention, the present method of modernisation further comprises the step of:
providing means for extracting and feeding the major portion of the gaseous flow comprising one or more passivating agents together with possible traces of ammonia and carbon dioxide in vapour phase from such at least one condensation unit to a washing unit provided downstream the reactor for urea synthesis.
In other words, through the introduction of suitable means, the major portion of the gaseous flow comprising the passivating agents fed by the stripping unit to the condensation unit is advantageously deviated to any one of the washing units located in the plant downstream of the reaction space instead of passing through the same.
Advantageously, the method of modernisation according to the present invention permits therefore to feed to the reactor for urea synthesis only a minor portion of the gaseous slow comprising the passivating agents, thus reducing to a minimum the presence of substances extraneous to the conversion reactionxe2x80x94to full advantage of the overall conversion yield that is increased by 1 to 3 percentage pointsxe2x80x94and at the same time guaranteeing a suitable protection of the reaction space against the corrosion.
In the same way as for the flow comprising ammonia and carbon dioxide in vapour phase, also in this case the term xe2x80x9cmajor portionxe2x80x9d is meant to indicate a portion greater than 50% of the overall flow of passivating agents coming out of the stripping unit.
For example, in case of a major portion comprised between 65-85% of the inert gas flow, only 15-35% of such inert gases is sent to the synthesis reactor, as compared to 100% for the prior art, with ensuing remarkable advantages in terms of greater conversion yield.
In this regard, it shall be noticed how it is praxis in the field to feed to the stripping unit a predetermined amount of passivating agents (for example, air or oxygen) together with the flow of carbon dioxide, in order to protect the apparatuses of the synthesis section of the urea plantsxe2x80x94and in particular the stripping unitxe2x80x94from a fast deterioration due to the corrosive action of the substances involved in such reaction. Therefore, the passivating agents and other possible inert substances, which may be contained in the flow of carbon dioxide, pass through the condensation unit(s) and are then sent to the reactor for urea synthesis.
As the operative conditions are less critical than for the stripping unit, the amount of passivating agents theoretically required for protecting the condensation section and the reaction space is substantially lower than that effectively circulating in the synthesis loop.
Consequently, both the processes for urea production with stripping in carbon dioxide, and the methods of modernisation of the plants for urea production according to the prior art, are influenced by a loss of conversion yield in the reaction space and therefore of a missing production capacity due to the content in excess of inert substances in such reaction space.
Thanks to the present invention, the increase in conversion due to a smaller amount of inert substances present in the reaction space together with the conversion increase due to the increase of the previously described overall reaction volume, permits to maximise the increase of conversion yield in the reaction space and therefore of the production capacity of the existing plant, without creating in this way imbalances or disorders downstream the reaction space, and in particular overloads of the existing condensation section.
Further on, it is worth mentioning that with the aforesaid increases of conversion yield, it is advantageously possible to maintain the energy consumption low and even, in some instances, to reduce the steam consumption with respect to the existing plant to be modernised.
The major and minor portions of the flow comprising ammonia and carbon dioxide in vapour phase respectively of the gaseous flow comprising one or more passivating agents is preferably adjusted according to this method of modernisation thanks to the step of providing suitable controlling means in the means for extracting and feeding the major portion of the gaseous flow comprising one or more passivating agents together with possible traces of ammonia and carbon dioxide in vapour phase from the condensation unit to such washing unit.
According to a further aspect thereof, the present invention relates to a process for producing urea of the type comprising the steps of:
reacting ammonia and carbon dioxide in a reaction space, obtaining a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution;
feeding the reaction mixture to a stripping section with carbon dioxide and subjecting the mixture to a treatment of partial decomposition of carbamate and partial separation of free ammonia, obtaining a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising urea and residual carbamate in aqueous solution;
feeding the flow comprising urea and residual carbamate in aqueous solution to an urea recovery section;
and which is characterised in that it further comprises the steps of:
feeding a minor portion of the flow comprising ammonia and carbon dioxide in vapour phase directly to the reaction space;
feeding a major portion of the flow comprising ammonia and carbon dioxide in vapour phase to at least one condensation unit and subjecting such major portion to a substantially total condensation, obtaining a flow comprising urea and carbamate in aqueous solution;
feeding the flow comprising urea and carbamate in liquid phase to the reaction space.
This invention further relates to a plant intended for carrying out the aforesaid process for producing urea, comprising:
a reactor for urea synthesis;
means for feeding ammonia and carbon dioxide to the reactor for urea synthesis;
a stripping unit with carbon dioxide for subjecting a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution leaving the reactor to a treatment of partial decomposition of carbamate and partial separation of free ammonia, obtaining a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising urea and residual carbamate in aqueous solution;
a recovery section of the flow comprising urea and residual carbamate in aqueous solution leaving the stripping unit for separating urea from the residual carbamate in aqueous solution;
and which is characterised in that it further comprises:
at least one condensation unit of the xe2x80x9csubmergedxe2x80x9d type for subjecting to substantially total condensation at least part of the flow comprising ammonia and carbon dioxide in vapour phase leaving the stripping unit, obtaining a flow comprising urea and carbamate in aqueous solution;
means for feeding a minor portion of the flow comprising ammonia and carbon dioxide in vapour phase leaving the stripping unit directly to the reactor for urea synthesis;
means for feeding a major portion of the flow comprising ammonia and carbon dioxide in vapour phase leaving the stripping unit to such at least one condensation unit;
means for feeding the flow comprising urea and carbamate in aqueous solution leaving said at least one condensation unit to the reactor for urea synthesis.
According to the invention, the plants intended for carrying out the process for urea production can be realised both ex-novo or by modifying pre-existing plants, so as to obtain an increase in the production capacity and in some cases an improved performance from the point of view of energy consumption.
Further features and advantages of the present invention will appear more clearly from the following non limiting description of an embodiment of the method of modernisation and of the urea synthesis process according to the invention, made with reference to the attached drawings.